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A previous CL (https://crrev.com/c/2001745) only sets the rounded
corners of shelf container when the ink drop of the app icon is
activated by gesture. However, it misses the case that during scroll
animation the rounded corners are also needed. This CL enables the
rounded corners during hotseat scroll animation.

In addition, this CL utilizes shelf button delegate to call the host
view's function from shelf app button. It improves the code readability.

Bug: 1055992
Change-Id: I59285bd693ab6e8833f1f82f9212c7a5195ce3ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2076538Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746188}